It cost me a while to analyze the cause of the following problem.
The symptom was that testing a local web app with twill was fast on Python 2.3, but very slow on Python 2.4-2.6 on a Win XP box. This boiled down to the problem that if you run a SimpleHTTPServer for localhost like this, BaseHTTPServer.HTTPServer(('localhost', 8000), SimpleHTTPServer.SimpleHTTPRequestHandler).serve_forever() and access it using httplib.HTTPConnection on the same host like this httplib.HTTPConnection('localhost', 8000).connect() then this call is fast using Py 2.3, but slow with Py 2.4-2.6. I found that this was caused by a mismatch of the ip version used by SimpleHTTPServer and HTTPConnection for a "localhost" argument. What actually happens is the following: * BaseHTTPServer binds only to the IPv4 address of localhost, because it's based on TCPServer which has address_family=AF_INET by default. * HTTPConnection.connect() however tries to connect to all IP addresses of localhost, in the order determined socket.getaddrinfo('localhost'). With Py 2.3 (without IPv6 support) this is only the IPv4 address, but with Py 2.4-2.6 the order is (on my Win XP host) the IPv6 address first, then the IPv4 address. Since the IPv6 address is checked first, this gives a timeout and causes the slow connect() call. The order by which getaddrinfo returns IPv4/v6 under Linux seems to vary depending on the glibc version, so it may be a problem on other platforms, too. You can see the cause of the slow connect() like this: import httplib conn = httplib.HTTPConnection('localhost', 8000) conn.set_debuglevel(1) conn.connect() This is what I get: connect: (localhost, 8000) connect fail: ('localhost', 8000) connect: (localhost, 8000) The first (failing) connect is the attempt to connect to the IPv6 address which BaseHTTPServer doesn't listen to. (This is the debug output of Py 2.5 which really should be improved to show the IP address that is actually used. Unfortunately, in Py 2.6 the debug output when connecting has even fallen prey to a refactoring. I think it should either be added again or set_debuglevel() is now pretty meaningless.) Can we do something about the mismatch that SimpleHTTPServer only serves IPv4, but HTTPConnection tries to connect with IPv6 first? I guess other people also stumbled over this, maybe without even noticing and just wondering about the slow performance. E.g.: http://schotime.net/blog/index.php/2008/05/27/slow-tcpclient-connection-sockets/ One possible solution would be to improve the TCPServer in the standard lib so that it determines the address_family and real server_address based on the first return value of socket.getaddrinfo, like this: class TCPServer(BaseServer): ... def __init__(self, server_address, RequestHandlerClass): if server_address and len(server_address) == 2: (self.address_family, dummy, dummy, dummy, server_address) = socket.getaddrinfo(*server_address)[0] else: raise TypeError("server_address must be a 2-tuple") BaseServer.__init__(self, server_address, RequestHandlerClass) ... That way, if you either serve as or connect to 'localhost', you will always consistently do this via IPv4 or IPv6, depending on what is preferred on your platform. Does this sound reasonable? Any better ideas? -- Christoph -- http://mail.python.org/mailman/listinfo/python-list